The pitch or spacing of rivets in the flanges The maximum end shear as above computed is taken by two stringers; and therefore the number of rivets required in bearing to form the connection between the stringers, connection angles, and the floor-beam web is, for each angle: The value 6 560 in the above equation is the value of a Finch field rivet in bearing in the Finch web.
The number of rivets required in the end angles on the floor beam is: These rivets go through the web of the floor-beam. The connection of the floor-beam to the end-post is made by means of field rivets and a_large gusset plate. This gusset plate is usually inch in thickness. The number of rivets through the end connection angles and this gusset plate is governed by single shear, since the rivets will shear off between the angles and the gusset plate before they will tear out of the gusset plate, as the value of a rivet is greater in bearing than in shear. The number required is: The general arrangement of the intermediate floor-beams is shown in Fig. 172. The ends of the lower flange are bent up as shown, in order to allow the I-bar heads or any other section of the lower chord to have clearance. This makes it necessary for the floor beam web to be spliced at the ends, as shown. The distance which this plate should extend above the floor-beam proper depends upon the distance which the lower chord is bent up. In any case the length of the connection on the post should be at least equal to the depth of the floor-beam. Two splice plates, one on either side of the web, are placed here in a manner similar to that of a splice as designed in the plate-girder when shear only was considered. liere shear only is considered, and the number of rivets which must be on each side of the splice will be: The 7 840 which occurs in the above equation is the value of a Finch rivet in hearing in a Finch plate (19). Inspection of Plate II (p. 172) will make this design clearer. Plate II also shows the shape of the end floor-beams.
The small shelf angle shown in Fig. 172 should have sufficient rivets to prevent any twist of the stringers due to their being con nected on one side of their web only. This number is a matter of judgment. Experience seems to indicate that enough rivets to take
up one-third of the total reaction of the stringers will be sufficient. This will require shop rivets, and the number will he: 103 180 3 X 7 220 5 shop rivets in single shear. ' 84. The Tension Members. Tension members usually consist of long, thin, flat plates with circular heads forged upon their ends.
These circular heads have holes punched through their centers and then very carefully bored. Through these holes are run cylindrical bars of steel called pins. These pins connect them with other mem bers of the truss. See Carnegie Handbook, p. 212, for table of I-bars. The I-bars given arc standard I-bars; and while departures from these widths and minimum thicknesses may be made, it may be done only at great cost to the purchaser. Note that there are no standard 9-inch I-bars. The thicknesses given are the minimum thicknesses for that width of bar, and do not indicate that thicker bars of that width cannot be obtained; but on the contrary thicker bars of that width can be obtained, and this should be done, the minimum thick ness as given in the table being avoided if possible.
It has been found that bars which have a ratio of thickness to width of about one-sixth give good service and are easy to forge. This relation gives us a rough guide which will enable us to determine the approximate width and thickness of any bar of a given area. Once the approximate dimensions are determined, the actual dimen sions can be chosen from the market sizes of the material (see Car negie Handbook, pp. 245 to 250).
An expression for the approximate depth of the bar will now be derived by using the above relation.
Let A = Area of bar, in square inches; d = Width of bar, in inches; t = Thickness of bar, in inches.
Then, Id = :1; also, u tiubstituting the value of in the expression for A, there results: The stresses in all the members in the truss under consideration are computed by the method described in Part I, and are placed on the stress sheet, Plate III (p. 251). In the succeeding design, the student should obtain his stresses from Plate III without his attention being again called to the matter.